On Tue, 18 Aug 2009, Rick L. Vinyard, Jr. wrote: > The read function pointer of the sysfs bin_attribute structure has this > signature: > > ssize_t (*read)(struct kobject *kobj, struct bin_attribute *attr, > char *buf, loff_t offset, size_t size); > > I've figured out the purpose of all the parameters except the loff_t > parameter. > > Obviously it's an offset of some sort, but what is the meaning of the offset? It's the same offset argument that's found in all read or write methods for all types of files. It refers to the offset from the start of the file. > If I have binary data in a char* named bindata of size bsize, should it be > copied into buf+offset in something like: > memcpy(buf+offset, bindata, bsize); > > Or, is it an offset from both buf and bindata in something like: > memcpy(buf+offset, bindata+offset, bsize-offset); Neither one. If the kernel wanted to add an offset to buf, it would have done so before calling the function. The code should be more like this: memcpy(buf, bindata + offset, min(size, bsize - offset)); Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
